In an image forming device using an electrophotographic process, foreign matter such as corona products, paper filler, and the like adhere to the surface of the photosensitive member through use over a long period of time; as a result of the drop in charging potential of the photosensitive member surface, developer adheres to non-image portions of the electrostatic latent image formed on the photosensitive member surface, resulting in the problem of the so-called fogging phenomenon, where developer is transferred to the non-image portions, which is to say blank portions, of the image transferred to the recording paper.
The following is known as a technology for suppressing this fogging phenomenon, for example: a technology comprising developing bias voltage control signal output means for outputting a developing bias voltage control signal, and a developing bias voltage varying circuit for varying the developing bias voltage of the developing means based on the developing bias voltage control signal output therefrom, the developing bias voltage of the developing means being raised or lowered according to key operations by a user. In another known technology, instead of key operations by a user, the developing bias voltage of the developing means is automatically raised or lowered based on a detected value of a density sensor for detecting the number of images recorded on recording paper, and the degree of fogging.
Another technology, for example, comprises density detecting means for detecting the density of a reference toner image formed on the photosensitive member; fog removal level input means (switch) for setting the fog removal level; first selecting means for selecting the optimal combination of the grid voltage for the charging means and the bias voltage of the developing means from predetermined combinations; and second selecting means for selecting one grid voltage from among a plurality of predetermined grid voltages with the grid voltage selected with the first selecting means as the standard corresponding to the fog removal level set with the fog removal level input means; wherein the grid voltage being changed according to the fog removal level set by the user. A technology is known where the fog removal level is set automatically according to the magnitude of the detected value from the density detecting means for detecting the density of the reference toner image formed on the photosensitive member, instead of the user setting the fog removal level using the fog removal level input means (switch).
However, in technologies such as described above, wherein the user inputs a change or a setting for the developing bias voltage or grid voltage with a key operation (switch operation) and the developing bias voltage or grid voltage is adjusted according to that input, the fogging phenomenon is temporarily resolved when the user who noticed the fogging phenomenon adjusts the developing bias voltage or grid voltage with a key operation (switch operation); however, when the operation of the image forming device continues, the fogging phenomenon occurs again and the user must again input the change or the setting of the developing bias voltage or grid voltage with a key operation (switch operation) when the fogging phenomenon occurs.
In technologies such as described above, wherein the developing bias voltage or grid voltage is adjusted automatically based on a detected value from a density detecting sensor or the like, it is necessary to provide a sensor for detecting the density of the standard toner image formed on the photosensitive member, a potential detecting sensor for detecting the surface potential of the photosensitive member, or the like; and the constitution of the device becomes complex, so that the cost becomes high.
The present invention was developed in view of these problems; an object thereof is to provide an image forming device which effectively suppresses the fogging phenomenon with a simple device constitution and which reduces the amount of work involved in the user inputting the adjustment to the charging output and the developing bias.